AC commercial power is used as a primary power source for computers and other data processing equipment which in turn use stored program and solid state technology. These circuits are generally very sensitive to input power variations from a desired standard waveform. However, commercial AC power waveforms are subject to many variations due to the demands of other users on the power line and other factors. Typical undesirable variations are over-voltage, under-voltage, voltage outages and signal transients. Undesirable variations also occur due to load conditions, as well as line conditions.
A UPS typically includes a battery backup as a secondary or reserve power source which is activated to supply power to the load upon total failure of the commercial power in the case of a blackout or significant degradation of the primary power source, as in a brownout. It is also known to use the regulation characteristics of ferroresonant transformers in UPS systems By way of further background, an exemplary UPS is disclosed in U.S. Pat. No. 4,475,047 (Ebert), granted Oct. 2, 1984.
FIG. 1 also illustrates a prior art uninterruptible power supply using a ferroresonant transformer as a constant voltage regulator. Commercial AC voltage for utility line operation and battery voltage "VB" for battery mode operation are regulated in amplitude by the ferroresonant transformer and the output voltage of the UPS is kept within a few percent, typically plus or minus four percent maximum, from its nominal value.
The battery and inverter "INV" do not deliver power to the output load if the UPS's AC input voltage is within predetermined limits. The inverter logic may be continuously operated in an idling condition, that is, the inverter logic is timed to operate so that the output signal of the inverter will be in phase with the output signal of the power supply. However, during idling, no power flows from the inverter to the output load. Thus the reserve power source is conditioned to respond immediately to supply power to the output load from the inverter upon failure of the primary source. On identifying a failure at the AC input line (the AC input is outside of predetermined upper and lower set points), the line side static switch "S/S" will be opened and the inverter and battery will be brought on the system, continuously supplying the output load.
Because this type of UPS system uses passive line conditioning techniques, the output voltage regulation of the system depends on the characteristics of the ferroresonant transformer and the input voltage range supplied to the ferroresonant transformer. Normally when designing a ferroresonant transformer, the input voltage range (between upper and lower set points) is determined for a full load condition to maintain the output voltage constant within about a four percent tolerance. Under a full load condition, if the input voltage is outside this predetermined range, the UPS should switch to the inverter and battery to prevent the UPS's output voltage from falling outside of its regulation range. However, due to the characteristics of a ferro-resonant transformer, if a load is less than a full load, the output voltage of the UPS could still be within the output regulation range even if the input voltage is outside the predetermined range.
FIG. 2 shows the ferroresonant transformer's regulation characteristics. The output voltage regulation depends on the output load of the UPS and an input voltage range to the UPS. For a full load condition, the output voltage can be maintained within a proper range if the input voltage is above 85% of its nominal voltage, above about 102 volts for 120 volts nominal. If the output load is a half load (50% of a full load), however, the output voltage will still be maintained within a proper range even when input voltage to the transformer is 70% of its nominal voltage, above about 84 volts for 120 volts nominal. Consequently, for different output loads, the input voltage switching point could be different while the output voltage is maintained within the regulation range. If the UPS uses a fixed lower switching point for input voltage, then precious battery power is consumed when a load that is less than a full load could be directly supported, within the regulation range, by the degraded AC input voltage.
The proliferation of computers and other data processing requirements has significantly increased the demand for and sophistication of UPS systems. Microprocessors are used in a UPS to obtain true sine wave inputs at precise phase and frequency, for example, a UPS of the type described in the aforementioned Ebert U.S. Pat. No. 4,475,047. Typically, however, even a sophisticated UPS is controlled as a function of input conditions. This causes switching from the primary power source to the reserve power source regardless of load conditions and thus creates an unnecessary drain on the battery.
It has been suggested in the literature that battery energy can be preserved using a microprocessor and an algorithm to dynamically modify the AC input line switch point according to both UPS output load current and AC input voltage variations. See "An Adaptive AC Monitoring Algorithm For Microprocessor Controlled Parallel Processing UPSs," H. E. Menkes, AT&T Bell Laboratories, Parsippany, N.J., 1987 IEEE publication CH2477-8/87/000-0512, IEEE INTELEC 1987 Conference Proceedings.
As noted by the aforementioned Menkes article, the primary power source is connected to the ferroresonant transformer through a solid-state line switch. The control circuitry must decide when the input switch should be open. In the prior art, this decision is based on the quality of the AC input to the UPS and the UPS's regulation characteristics as described hereinabove. In practice, for example, according to the Menkes article, for a nominal 208-volt AC input voltage, the threshold for operating the UPS input line switch would be approximately 184 volts AC at the low end and 220 volts AC at the high end. The Menkes article concludes that the fixed low end set point dictates that the input line switch would be open upon a slight brownout condition, even though not necessarily required depending upon the load and the regulation characteristics of the UPS. The UPS would treat a slight brownout as a full blackout and unnecessarily consume battery energy. In the case of a brownout, as with a full blackout, battery energy could be fully expended, particularly for small UPSs with short battery reserve times of approximately five minutes. The Menkes article proposed to overcome this problem by monitoring the condition of both the AC input and the load in order to make decisions on the set point at which the input switches would be opened using an adaptive algorithm requiring repeated calculations and decisions. Menkes concluded that it was "impractical and unnecessary" to program the microprocessor to track the regulation characteristics, line and load exactly. Hence the algorithm used a stepwise approximation.
A stepwise approximation could be useful for applications where the load conditions are such that they are not very sensitive to input variations, for example telephone switch gear. However, for loads that are more sensitive to input variations, the stepwise approximation has several disadvantages. Although the Menkes article suggests five stepwise approximations for a nominal 208-volt input UPS, theoretically the number of steps in the approximation could be increased. This would lead to a complex implementation that would not be practical and cost-efficient and could even require a dedicated microprocessor for this limited purpose while requiring further microprocessors to perform the other control functions required by the UPS. With only a limited number of stepwise approximations, the system will not provide optimum conservation of the battery because it would be insensitive to variations within each step and it could be overly responsive to small variations between adjoining steps. With stepwise approximations, even if load conditions would not require a switch to reserve power, the system will still switch to reserve power. Moreover, as noted in the Menkes article, step load increases while in the adaptive mode of operation will not cause immediate switching to the inverter, but rather creates a delayed response until the next time the program loop is executed.
It is desirable, particularly for brownout conditions, not to switch to reserve power unless it is absolutely required by the load conditions. However, the adaptive algorithm technique proposed by the Menkes article is operative only after the UPS has switched to the inverter mode and after verification that the AC input voltage variation is due to a brownout condition and not a blackout condition. For the embodiment described in the Menkes article, whenever the input voltage is less than 184 volts, the UPS first switches to the inverter, regardless of load. Also, the adaptive algorithm is turned off at input voltages above 190 volts pending a decision to enable or disable the adaptive portion of the algorithm. This creates the further possibility of switching to the inverter, regardless of load, if there is a short brownout. Menkes does not consider output voltage in determining input voltage switching points.
By way of further background to the present invention, fuzzy inference controllers are also known for certain applications, for example see "Fuzzy and Neutral Network Controller", Yasuhiko Dote, 1990 IEEE paper No. 087942-600-4/90/1100, pages 1314-1329 published in IECON, 1990, 16th Annual Conference of IEEE Industrial Electronics Society held Nov. 27-30, 1990, Volume II, Publication No. 90CH2841-5. Specific examples of fuzzy inference logic controllers are also described in U.S. Pat. No. 5,176,858 (Tsukabe, et al.), granted Jan. 5, 1993, for a method and apparatus for controlling molding machines and U.S. Pat. No. 5,104,109 (Kubo), granted Apr. 14, 1992, for a paper sheet delivery/stacking control system. However, prior to the present invention, others have failed to apply fuzzy logic to a UPS utilizing the regulation characteristics of a ferroresonant transformer. It might be surmised that this failure occurred because UPS design is conventionally concerned with precise control whereas fuzzy inference control may have been considered too imprecise for that application.